The IEEE 802.3/ETHERNET standard is the most popular physical layer LAN protocol in use today. Networked computing hardware incorporating and carrying out ETHERNET protocols are now utilized within industrial process control/factory networks. Such networks connect a variety of process control data sources, including control processors, to monitoring stations. Such local area networks facilitate visualization and control of industrial processes through the monitoring stations. By way of example, data received by a monitoring station drives a process visualization application providing visual output representing a current state of a plant process (e.g., temperature, pressure and/or fluid levels within tanks, mixers, reactors, etc.). Based upon such displayed information, a plant manager potentially initiates one or more of a variety of supervisory control operations (e.g., re-setting a set point, shutting down a process, etc.) with regard to the monitored plant process.
A number of ETHERNET technology enhancements build upon the basic IEEE 802.3/ETHERNET standard—including ones that support redundant, multi-path physical connectivity between nodes of a local area network infrastructure. Such ETHERNET network technology enhancements include: Rapid Spanning Tree (IEEE 802.1w), Ethernet Automatic Protection Switching (RFC 3619), Full-duplex (IEEE 802.3x), and high speed Ethernet (IEEE 802.3u, 802.3z, 802.3ae). These improvements have facilitated deploying ETHERNET LANs as highly reliable deterministic self-healing redundant networks that support bi-directional communications between sources of industrial process data (e.g., data access servers, embedded process controllers) and supervisory-level monitors for observing and controlling plant processes at a supervisory level. Furthermore, such enhancements enable redundant data path fail-over in less than a second.
In many instances nodes execute critical supervisory process control/visualization applications. Such applications present a supervisory view of an industrial process, report alarms/events, and expose controls for enabling an engineer to respond quickly to such alarms/events—even execute logic to render automated supervisory control instructions. Even in a high availability system network incorporating one or more of the above-mentioned ETHERNET technology enhancements, a node without redundant network connection access points (also referred to as ports) is a single point of failure. Therefore, to enhance reliability of communications between nodes running critical process control/visualization applications in a plant process control network, redundancy has been extended to the physical connections between such nodes and physical LANs.
In nodes having redundant network connections to a LAN, loss of a primary access point/port connecting a redundantly configured node to the LAN is detected and a transparent switch is made to the backup access point/port. One type of network connectivity test (referred to herein as end-to-end connectivity) involves confirming network connectivity between two endpoint nodes on a network that seek to communicate with one another. End-to-end connectivity between two nodes has been detected by data link (layer two) processes. For example, currently pending Burak, U.S. patent application Ser. No. 10/775,633, entitled “System and Method for Ethernet Redundancy,” discloses a technique for detecting the status of a node's redundant network connection physical access ports and transparently switches from a primary network access port to a backup access port upon detecting an end-to-end failure of the primary access port. The Burak '633 application discloses performing end-to-end failure detection by layer two processes for each packet transmission according to the IEEE 802.2 Logical Link Control Type 2 or Type 3 (i.e., LLC2/LLC3) data link protocols. Therefore, in connections incorporating the above-described redundancy capabilities, when a failure in a network path between two communicating nodes is detected by a node having redundant connections to a network, the node switches its point of network access from a current primary physical port to a current backup port.
The above-mentioned end-to-end network connectivity fault detection and primary port switching techniques are intended for use in networks where network paths between two nodes are potentially incapable of deterministic healing—i.e., a path fault potentially persists for an indefinite time period. In such network environment, after a network path failure has occurred and the network potentially commences a healing process, a node retransmits its packet up to “r” times, in accordance with a reliable data link protocol, to ensure the network data path had indeed failed. If the node is unsuccessful in each of the “r” transmissions of a packet to a destination node, then the node switches its primary physical network access point to the current backup physical network access point. After switching the physical network access point, the node retransmits its packet up to “r” times using this alternate network access point. If this second set of “r” attempts fails, then the node switches back to the previous access point (for “r” more packet transmission attempts). The sets of “r” retransmissions (one set per redundant physical network access point/port) are repeated up to “n” times—ending retransmissions when the network self-heals and/or the packet is successfully received by the destination node. U.S. Pat. No. 6,173,411 discloses periodic end-to-end connectivity testing to detect end-to-end connectivity failures between network access points/ports.
The previously-mentioned enhanced ETHERNET protocols (e.g., Rapid Spanning Tree Protocol) facilitate deploying LANs with highly available, deterministic self-healing (in less than a second), network paths between attached nodes. In deterministic self-healing networks, re-configurable switches between a source node and destination node are reconfigured, in the event of a path failure, to re-route packets on a substitute path between two communicating nodes. As a consequence there is no need for the nodes themselves to implement data link layer protocols for detecting end-to-end network connectivity (since such connectivity is ensured, if at least one such path exists, by the network itself). Self-healing networks do not provide any special advantage in cases where a presently selected access point/port fails. Furthermore, a link failure may render a portion of a network inaccessible through one of a set of redundant access points.